In fuel cell systems, anode concentration at start-up is a key parameter for start/stop controls to meet emissions standards. Currently, anode concentration is determined by Beginning of Life (BOL) characterization from a lookup-table (LUT). However, BOL characterization may not be representative of an accurate anode concentration over time.
In current state-of-the-art production-focused fuel cell systems (FCS), system start-up is relatively inefficient in regards to hydrogen usage. Current FCS start-up methods typically include a stack anode header purge procedure and a stack anode flush procedure to prevent electrode catalyst degradation. The purge and flush procedures vary in duration and flow rate based on how long the FCS has been off (e.g. using a system “off-timer”). If the FCS has been off for a short time, it is assumed that a large amount of hydrogen remains in the stack. As the system ages, balance-of-plant components (such as valves) and the stack membrane can degrade. Such degradation can increase the rate of hydrogen diffusion from the anode. Because of this known behavior, the current start-up methods are calibrated for an end-of-life FCS. Therefore, the start-up method may implement a header purge when one is not necessary. Furthermore, a beginning-of-life FCS with exceptional (high-end) components and/or membranes may have an extremely slow hydrogen diffusion rate (slower than the calibration benchmark); in this case, a much longer off-time may not require as long a header purge or flush.
At the present time, there exists no high-performance (˜ ms response time) production-feasible hydrogen concentration sensor for use internally in the FCS. With such a sensor, the passing of a hydrogen front through the header purge or flush valve could be detected, wherein the hydrogen front denotes a desired end-point of the header purge procedure or header flush procedure. Additionally, a means to detect the composition of a fluid in the FCS would make it possible to modify start-up procedures to ensure optimum operation.
It would be desirable to develop a fuel cell system and a method for determining a composition of a fluid in the fuel cell system, thereby providing a basis to tune and modify a start-up procedure of the fuel cell system to maximize efficiency, durability, and reliability of the fuel cell system from beginning-of-life to end-of-life.